Corrosion & its prevention

Most metals can corrode (rust) in the presence of oxygen to form the corresponding metal oxide. If the metal is in the presence of water and oxygen, it is known as rusting.

iron + water + oxygen —> rust

Over time, iron rusts and its structure becomes weakened.

Investigating Rusting

Oxygen and water MUST be present for rust to occur. Boiled water removes any dissolved oxygen and calcium chloride is a drying agent. Three test tubes are set up: 1) nail in air and water 2) nail in water only 3) nail in air only.

In this experiment, the nail only rusted in air and water and didn’t rust in only water or only air. This experiment proves that both air and water must be present for rusting to occur.

Rust Prevention

Barrier Methods - coating iron with barriers that prevent iron from coming into contact with oxygen and water. if the coating is washed away/scratched, the iron is exposed and will rust. Common barrier methods include electroplating, paint, oil and grease. Once iron begins to rust, it corrodes internally since rust is porous and allows both air and water to come into contact with fresh metal underneath.

Sacrificial Protection - iron can be protected by using a more reactive metal to coat iron. This however needs to be regularly replaced.

Galvanising - this is a process where iron is coated with a layer of zinc. This can be done by electroplating or dipping it into molten zinc. zinc carbonate is formed when zinc reacts with oxygen and carbon dioxide which protects the iron by the barrier method. if the coating is damaged or scratched, the iron is still protected from rusting by sacrificial protection.

Aluminium - oxide coating that prevents metal from further corrosion. Oxide layer acts as a barrier.